1. Field of the Invention
The present invention relates to a structure and process for reducing variation in interconnect parasitic capacitance, and in particular, to a process and apparatus utilizing insertion of a third metal line between adjacent metal lines to reduce interconnect parasitic capacitance variation.
2. Description of the Related Art
The ever-decreasing feature size of semiconductor devices, and the corresponding increase in packing density, has rendered integrated circuits (IC's) more sensitive than ever to signal propagation delays. At this advanced phase of IC development, IC operation is limited by the delay in propagation of signals between active devices of the circuit, rather than by the speed of the semiconducting devices themselves.
Propagation delay is determined in large part by parasitic resistive-capacitive (RC) delay caused by interconnect linking together various devices of the IC. The magnitude of this RC delay is in turn determined in large measure by the parasitic capacitance (CPAR) component.
In designing IC's, engineers can and do take parasitic capacitance into account. However, this task is complicated by the fact that interconnect parasitic capacitance varies between maximum and minimum values. Therefore, the engineer must ensure that the IC can function over the entire range of variation in interconnect parasitic capacitance.
One important source of variation in interconnect parasitic capacitance is the variation in critical dimension (CD) of adjacent metal lines of an interconnect metallization layer. This is illustrated in FIGS. 1A-1C.
FIG. 1A shows a cross-sectional view of an interconnect 100 featuring interconnect metallization layer 102 including adjacent first and second metal lines 102a and 102b respectively. Metal lines 102a and 102b are positioned over lower interlayer dielectric (ILD) 104. First and second metal lines 102a and 102b are formed by patterning a photoresist mask over interconnect metallization layer 102, and then etching interconnect metallization layer 102 in unmasked areas to stop on underlying lower ILD 104. Next, a second interlayer dielectric 106 is formed over the entire surface, such that dielectric material 106 penetrates into inter-line region 108 between metal lines 102a and 102b. 
Parasitic capacitance arising between first metal line 100a and second metal line 100b obeys the following equation:CPAR=(εS)/d, where     CPAR=parasitic capacitance;    ε=dielectric permittivity;    S=area of the plates of the capacitor; and    d=distance between the adjacent metal lines.
Variation in interconnect parasitic capacitance can be introduced during fabrication of the interconnect structure. One source of parasitic capacitance variation occurs during photolithography leading to formation of the metal lines. Specifically, variation in width of the patterned photoresist mask can in turn induce variation in parasitic capacitance.
This is illustrated by FIGS. 1B and 1C, which also depict cross-sectional views of adjacent metal lines of an interconnect metallization layer.
In FIG. 1B, variation in photolithographic processing has led to formation of adjacent metal lines 102a and 102b possessing a width narrower than that of the adjacent metal lines depicted in FIG. 1A. Because of this changed critical dimension, the distance between adjacent metal lines 102a and 102b is increased. And, as a direct consequence of Equation (I), the corresponding parasitic capacitance is reduced.
Conversely, FIG. 1C shows a cross-sectional view of adjacent metal lines of an interconnect metallization layer wherein photolithographic processing has created metal lines 102a and 102b wider than the adjacent metal lines of FIG. 1A. As a result of this changed critical dimension, the distance between adjacent metal lines 102a and 102b is decreased, and the corresponding parasitic capacitance is increased.
The relation between variation in critical dimension and interconnect parasitic capacitance is shown in FIG. 2. FIG. 2 plots variation in critical dimension (ΔCD) versus parasitic capacitance (CPAR). FIG. 2 shows that ΔCD introduces a spectrum of possible parasitic capacitances into an interconnect structure. This capacitance variation CVAR ranges between a minimum capacitance (CMIN) wherein ΔCD is a negative value (and adjacent metal lines are narrow), and a maximum capacitance (CMAX) wherein ΔCD is a positive value (and adjacent metal lines are wide).
Because variation in parasitic interconnect capacitance governs anticipated signal propagation delay and thereby confines design of IC's, there is a need in the art for an interconnect structure and a process for forming an interconnect structure wherein variation in parasitic interconnect capacitance is minimized.